1. Field of the Invention
The present invention relates generally to flow field management. More specifically, the present invention relates to systems for manipulating fluid flows utilizing synthetic jet actuators.
2. Description of the Related Art
Adverse (pressure gradient) fluid flows generated over aerodynamic surfaces can buffet and fatigue any downstream structures so exposed. Additionally, such flows can affect efficiency by increasing drag or resistance over the surface. Such adverse fluid flows can be generated at the fore body of an aircraft or other upstream structure, and damage control surfaces, engines, after body/empennage, nacelles, turrets, or other structures integrated into the airframe. Additionally, these adverse fluid flows can be ingested within engine air intakes or other like air inlets leading to poor performance and/or stalling of the aircraft engines. Stalling the aircraft engine creates a potentially hazardous condition. Next generation aircraft, such as blended wing body, compound this problem by incorporating gas turbine inlets with serpentine spines within the air frame. Additionally, exotic aperture shapes for the inlet and outlet may cause excessive propulsion performance losses. These losses emanate from strong secondary flow gradients in the near wall boundary of the airflow, which produce coherent large-scale adverse fluid flows.
In the past, aircraft components were designed to minimize the strength of adverse pressure gradient flow fields to reduce the extent of or eliminate the separation of boundary layer flow from aircraft surfaces to reduce the destructive structural impact of separated flow on aircraft components and performance. This approach limits design options and increases vehicle size, weight and cost. Alternatively, the components in the path of the adverse fluid flows were structurally hardened or replaced more frequently to avoid failures resulting from these stresses. Placing components, such as engines or control surfaces, in non-optimal positions in order to reduce these stresses often results in reduced vehicle performance. Similarly, adding structural weight to support increased stress loads caused by the flow field vortices also results in reduced vehicle performance.
Other solutions include the employment of active or passive control flows through mass injection using positive and/or zero mass devices to mitigate the effects of the adverse flow fields. These control jets manipulate the boundary layer, for example, through induced mixing between the primary fluid flow and the secondary fluid flow. The mixing is promoted by vortices trailing longitudinally near the edge of the boundary layer. Fluid particles with high momentum in the stream direction are swept along helical paths toward the aircraft surfaces to mix with and, to some extent replace low momentum boundary layer flow. This is a continuous process that provides a source to counter the natural deceleration of the flow near a solid surface in a boundary layer that can lead to flow separation in regions with adverse pressure gradients and low energy secondary flow accumulation.
It has been found that mass injection and other flow control devices can be used in place of mechanical flight or other vehicle controls. Mass injection devices utilizing a positive mass flow include, for example, passive jet spoilers which can utilize engine bleed air, ram air from an inlet or scoop, or a air/fluid pump. Such devices, however, require pneumatic/fluid conduits and/or manifolds to bring the control jets to regions requiring flow-control authority. Additionally, utilization of such devices result in added structural weight to supply and support the control jets, which results in reduced vehicle performance.
Various other types of positive mass flow devices include combustion-driven jet actuators, which oxidize a gaseous fuel-air mixture. Specifically, such combustion-driven jet actuators include a combustion chamber that is filled with a combustible mixture which is then ignited, resulting in high pressures inside the chamber and mass expulsion through a chamber orifice. Besides the necessary fuel and air conduits, such devices also require a fuel storage capability, mechanical valves, and a means for igniting the fuel, which result in added structural weight to supply and support the control jets, which results in reduced vehicle performance.
Zero mass flow-capable devices include mechanical synthetic jets, single or dual bimorph synthetic jets, and spark jets. Synthetic jets, for example, which may be large scale devices or small scale Micro-fabricated Electro-Mechanical Systems (MEMS) devices, can be employed along an airfoil surface to control flow separation on the airfoil. A typical synthetic jet actuator includes a housing forming an internal chamber and an orifice in a wall of the housing. The actuator further includes a mechanism in or about the housing for periodically changing the volume within the internal chamber so that a series of fluid vortices are generated and projected into an external environment flow beyond the orifice of the housing. Various volume changing mechanisms include, for example, a reciprocating piston configured to move so that fluid is moved in and out of the orifice during reciprocation of the piston, and/or a flexible diaphragm forming one or more walls of the housing. In a similar device, the flexible diaphragm can instead be actuated by a piezoelectric actuator, such as, for example, one or more bimorph piezoelectric plates or other appropriate means connected by a flexible hinge or hinges. Regardless of the configuration, the fluid moved may be either a liquid or gas, depending upon the state of the operational environment.
Mechanical and bimorph synthetic jet actuators employing a flexible diaphragm typically include a control system is to create time-harmonic motion of the diaphragm. As the walls of the diaphragm (or diaphragms) move into the center of the chamber, the chamber volume decreases, and fluid is ejected from the chamber through a chamber orifice. As the fluid passes through the orifice, the flow separates at the sharp edges of the orifice and creates vortex sheets which roll up into vortices. These vortices move away from the edges of the orifice under their own self-induced velocity. As the vortices travel away from the orifice, they synthesize a jet of fluid, a “synthetic jet,” through entrainment of the ambient fluid. As the walls of the diaphragm move outward with respect to the center of the chamber, increasing the chamber volume, ambient fluid is drawn in from large distances from the orifice and into the chamber. Notably, the inventors have found that such synthetic jet actuators, in general, and the dual bimorph synthetic jet actuators, in particular, are especially well-suited at low, medium, and relatively high jet velocities and where fine flow control is needed.
The other aforementioned zero mass-capable device, a spark jet, can also be employed, for example, along an airfoil surface in a similar fashion to that of the mechanical or bimorph synthetic jets to control flow separation on the airfoil. Akin to the mechanical or bimorph synthetic jets, a typical spark jet also includes a housing forming an internal chamber and a chamber orifice in a wall of the housing. In contrast to the mechanical or bimorph synthetic jet actuators, however, the spark jet includes electrodes to produce an electrical discharge to heat the fluid within the internal chamber, which causes the fluid to accelerate out of the chamber orifice. The walls of the spark jet are generally relatively rigid in order to withstand the chamber pressure resulting from the rapid heating of the fluid within the chamber, without significantly deforming. The inner chamber pressure is relieved by the exhaustion of the heated fluid through the chamber orifice. Fluid is returned to the inner chamber through a corresponding decrease in pressure caused by cooling of the chamber walls and the gases remaining within the internal chamber upon removal of the current to the electrodes. Notably, the inventors have recognized that although the maximum frequency of the spark jet is typically less than that of the typical bimorph synthetic jet due to its dependence upon the speed of cooling of the chamber between cycles, the rise time associated with the generation of operational pressure and mass flow during each spark jet cycle can be less than that of conventional synthetic jet actuators. The inventors have further recognize that such capability, if harnessed, could be utilized to enhance the performance of an airfoil utilizing solid-state synthetic jet actuators for primary flow control, particularly where changes in jet velocities and large flow disruption may be periodically needed, which approach or exceed the capability of the solid-state synthetic jet actuator.
Accordingly, the inventors have recognized that there is a need for flow control systems, apparatus, devices, controllers, program product, and methods which provide the advantages of both the solid-state synthetic jet actuator and the spark jet. Particularly, the inventors have recognized there is a need for flow control systems, apparatus, devices, controllers, program product, and methods which utilize the concepts of a spark jet to selectively enhance/extend performance of a dual bimorph synthetic jet, such as, for example, when maximum performance is desired.